http://www.ghoststudy.com/a_what_are_orbs.html 

Orbs are believed (by many) to be ghosts in the form of balls of light. They are life forms that travel in groups and are believed to be the human soul or life force of those that once inhabited a physical body here on earth. Psychics claim to talk to them on a regular basis, and ghost hunters encounter them quite frequently. It is said that they are those spirits that have willingly stayed behind because they feel bound to their previous life or previous location for whatever reason. Because of this obsession they tend to become similar to a psychotic human beings. It should be said that the majority of us when we die proceed gladly and willingly to the next level of existence after saying our quiet good-byes, which means we're off to the spirit world. Then again, as stated, a select few elect to stay behind because of a refusal to move on. Apparently the longer they stay behind, the harder it is to find their way to the next level, which again, is the spirit world.

Ghostly orbs are the most photographed anomalies caught on film by ghost hunters and are quite photogenic (when they want to be). They can be completely transparent or display themselves in a bright solid form. It is not hard to capture them on film in their circular form. It is theorized that ghosts prefer the form of an Orb (ball of light) because it takes less energy thus being the mode of choice among the ghosts. The consensus is that small orbs take up the least amount of energy and apparitions and other fuller shapes take up the most. In the colder parts of the year, it seems that ghosts tend to find it easier to take on shapes other than orbs. That's when there is the most static electricity in the atmosphere. Those months being October through February. It is also believed that they are able to draw on our own energy when needed.  I've listed these ways but there are obviously many ways for ghosts to draw energy from the atmosphere and environment  but I won't take the time for that here.

You can get more ghostly definitions at:

 http://www.the-atlantic-paranormal-society.com/articles/general/glossary.html